1. Field of the Invention
The invention relates to apparatus for balancing an asymmetrical load which is fed from a three-phase network.
2. Description of the Prior Art
Balancing devices of the above type are known in the prior art. One particular type of device comprises a self-commutating converter which is adapted to be connected on its a-c side with its phases interchanged to the three-phase network and on its d-c side to a d-c source. The converter includes controlled valves which are arranged in a three-phase bridge circuit, have provisions for forced quenching and are controlled such that an output current is generated having a line-frequency fundamental whose phase relative to the phase of the network a-c voltage and whose amplitude are dependent on the phase and the amplitude of the countercurrent system required for balancing. This type of balancing device is disclosed in the German Offenlegungsschrift No. 2,247,819.
In order to avoid or minimize reactions on the three-phase network due to an asymmetrical load, such as, for example, an arc melting furnace, the reactive power of the load must be compensated and the asymmetrical distribution of the active load over the individual network phases must be balanced. As the reactive power and the asymmetry of the load can change almost instantaneously, the apparatus used for compensating and balancing must have a control time in the range of milliseconds so as to be able to adequately regulate the rapidly changing reactive power and asymmetry.
As is well known, the system of currents caused in a three-phase network by an asymmetrical three-phase load can be resolved into a symmetrical corotating and a symmetrical counter-rotating current system. If a corresponding corotating current system is fed into the three-phase network, the reactive power due to the load can be compensated. Furthermore, by feeding-in a corresponding counterrotating current system, the asymmetry power of the load can be compensated. Operation of the above-mentioned balancing device of German Offenlegungsschrift No. 2,247,819 is based on the aforesaid principles.
More particularly, in the known balancing device of German Offenlegungsschrift No. 2,247,891, a self-commutating converter is connected to the three-phase network with its phases interchanged via a transformer. The d-c source supplying d-c current to the converter is further in the form of a line-commutated converter connected to a smoothing choke. Both the line-commutated and the self-commutating converters are timed by the line frequency. As a result, a common measuring arrangement for the reqired countercurrent system can be provided for the line-commutated and the self-commutating converter. With this measuring arrangement, the magnitude of the balancing countercurrent system can be influenced via control of the line-commutated converter and the phase sequence via control of the self-commutating converter.
In this aforesaid known balancing system, if the countercurrent system is to be changed, the output current of the line-commutated converter must be changed via the control pulses thereof. However, the smoothing choke linking the line-commutated converter to the self-commutating converter, due to its appreciable inductance, presents considerable inductive reactance to fast current changes thereby slowing them down. As a result, only relatively slow output current changes of the self-commutating converter can be achieved with a given rated power of the line-commutated converter. If the apparatus is to be capable of handling fast changes in the asymmetry power, then the line-commutated converter must be designed for high voltages and also for large currents; i.e., the converter must be designed for a very large power rating. In such case, the reactive power consumed by the line-commutated converter will be very considerable.
It is an object of the present invention to provide a balancing apparatus for the above type which is designed to permit compensation for rapid asymmetry changes, while not being excessively costly or consuming excessive amounts of reactive current.